This invention relates generally to methods for coating a moving web and, more particularly, to controlling adhesion levels between the moving web and the coating applied thereto, and most particularly, to controlling adhesion levels between photographic film base web and the multilayer coatings applied thereto.
Multilayer coating is well known in the prior art as described, for example, in U.S. Pat. No. 2,761,791 to Russell. In the method typically referred to as slide bead coating, a multilayer composite comprised of superimposed individual liquid layers is delivered to the moving substrate through the use of a coating die. At the end of the coating die, the layers form a continuous liquid bridge or coating bead between the die and the moving substrate. The slide bead coating method is useful for making thin, highly uniform, composite elements suitable for numerous applications, including photographic, thermographic, x-ray, and photoelectric films, among others.
Various methods are known in the prior art to promote adhesion between the support and applied layers. In most cases, an adhesive layer or subbing layer is required on a support prior to application of the functional layers. There are a number of disclosures describing unique chemical compositions suitable for subbing layers. U.S. Pat. No. 4,748,150 to Vanier teaches the use of a vinylidene chloride copolymer as a subbing layer to promote the adhesion of a subsequent layer containing a polycarbonate. Alternatively, a support may be subjected to electrical discharge pretreatments including corona discharge as described in U.S. Pat. No. 3,376,208 to Wood, glow discharge as described in U.S. Pat. No. 5,425,980 to Grace, or electron-beam irradiation U.S. Pat. No. 4,594,262 to Kreil. More commonly, a substrate requires a combination of electrical treatment and the application of a subbing layer to achieve good adhesion such as, for example, described for example in U.S. Pat. No. 4,128,426 to Ohta, U.S. Pat. No. 4,689,359 to Ponicello, and U.S. Pat. No. 5,232,825 to Hattori.
The equipment required for electrical treatment of substrates, such as those described above, is expensive and adds complexity to the manufacturing process. Moreover, such treatments may not be suitable for some coatings when layers are sensitive to chemical alteration of the support by electrical treatment. Similarly, the materials required for an effective subbing layer may be expensive or difficult to identify. There are no known coating methods describing enhancement of adhesion of a functional layer to the support.
Finally, it is sometimes desirable to attenuate the adhesive strength of a functional layer to the support depending on product requirements. In most cases, strong adhesion between applied layers and support is needed. However, some products, such as those designed to delaminate during usage, may require only weak adhesive strength. Continuous modification of the chemistry of the subbing layer to achieve such aims may be expensive and time consuming.
It is therefore an object of the present invention to provide a method for controlling the level of adhesion between a web and a coating applied thereto.
It is a further object of the present invention to provide a method for applying multilayer coatings to moving webs which allows for modification of adhesion levels between the moving web and the multilayer coating.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by selecting the support web with or without a subbing layer applied thereto; selecting an organic solvent that is substantially free of other constituents as a carrier layer; forming a multilayer composite including the carrier layer, the carrier layer being the lowermost layer thereof; applying the multilayer composite to the support web or the subbing layer; drying the multilayer composite on the support web, the carrier layer evaporating from the multilayer composite; testing the adhesive strength between the support web or subbing layer and the multilayer composite after drying; and repeating these steps until a desired level of adhesion is found. The method of the present invention may be practiced in conjunction with slide bead coating apparatus, extrusion hopper coating apparatus, and curtain coating apparatus. In a slide bead coating operation, the multilayer composite flows down the slide surface of the slide bead coating hopper and over a coating lip of the coating hopper to form a coating bead bridging the gap between the lip in the moving web. In this manner, the multilayer composite is applied to the support web or the subbing layer. The carrier layer preferably has a viscosity in the range of 1-10 cp and is applied at a wet thickness in the range of 1-20 xcexcm; flowing the multilayer composite down the slide surface and over a coating lip of the coating hopper; and forming a coating bead between the coating lip and the web.
In the practice of the method of the present invention, one or more of the upper layers preferably has a viscosity that is greater than the viscosity of the carrier layer. When the carrier layer is formed of a blend of organic solvents, preferably at least two of the following components: methanol, ethanol, isopropanol, n-butanol, acetone, methylethyl ketone, methylisobutyl ketone, toluene and methylene chloride are contained in the blend. It is also preferred that the layers above the carrier layer contain at least one of the following polymeric materials: cellulosics, polyvinylbutyrals, polycarbonates, polyurethanes and polyesters. Further, in the practice of the method of the present invention, it is preferred that the layers above the carrier layer have a combined wet thickness in the range of range of 1-500 xcexcm and at least one of such upper layers has a viscosity in the range of 10-5,000 cp. In addition, one or more of such upper layers may include water.
As mentioned above, the present invention allows for the control of the level of adhesion between the multilayer composite and the web such that the physical bond therebetween is strong or weak. In particular, the use of the present invention is shown to substantially improve the adhesion of a functional layer to the substrate. Moreover, this advantage is achieved without chemical modification of the support through the use of electrical discharge treatments. Finally, this advantage is achieved without the use of permanent chemical additives in the coated layer. Such additives may be expensive or detrimental to the properties of the final film.
Although the present invention is discussed herein with particular reference to a slide bead coating operation, those skilled in the art will understand that the present invention can be advantageously practiced with other coating operations. For example, the control of the level of adhesion should also be achievable with multilayer extrusion hopper coating operations and multilayer curtain coating operations. In addition, those skilled in the art will recognize that the present invention can be advantageously practiced with carrier layers containing additives such as polymers, surfactants, and etching agents. Practical applications of the present invention include photographic, thermographic and x-ray films as well as photographic, thermographic and inkjet papers, among others.